A light transmitting and receiving module transmits and receives light signals of a single channel (single wavelength) in, for example, a wavelength-multiplexing (WDM) light communication system. FIG. 6 illustrates an example of a WDM light communication system having a light transmitting and receiving module used in a WDM transmission apparatus. A WDM light communication system 100 illustrated in FIG. 6 is coupled to an opposing WDM transmission apparatus 103 via a WDM transmission path 101 and a light amplifier/relay apparatus 102.
A WDM transmission apparatus 103 transmits and receives a WDM light in which signal lights of respective channels (wavelengths λ1˜λn) are wavelength multiplexed, and includes light transmitting and receiving sections 112-1 through 112-n which are dedicated one to each channel and each of which includes two transmitting and receiving modules 111 serving as a duplex component, redundancy multiplexing/demultiplexing sections 113-1 through 113-n which are dedicated one to each channel, a light multiplexing/demultiplexing section 114 which multiplexes signals of respective channels into WDM light and which demultiplexes WDM light into signals of respective channels, and a light amplifier 115.
Individual transmitting and receiving modules 111 included in each light transmitting and receiving section 112 have respective light transmitting and receiving circuits 116 each of which includes, besides the configuration of transmitting signal light, photodiode which receives signal light received in the transmitting and receiving module 111, and respective switches 117 that switches between the working transmitting and receiving module 111 and the protection transmitting and receiving module 111. Namely, respective light transmitting and receiving circuits 116 in transmitting and receiving modules 111 serving as a duplex component output transmitting signal light. The switch 117 of the transmitting and receiving module 111 set to be the working module outputs a transmitting signal light from the light transmitting and receiving circuit 116 to the downstream redundancy multiplexing/demultiplexing section 113 (switch on) while the switch 117 of the transmitting and receiving module 111 set to be the protection module disconnects a light path to the redundancy multiplexing/demultiplexing section 113 (switch off). A receiving signal light is received in respective light transmitting and receiving circuits 116 of the transmitting and receiving modules 111 serving as a duplex component and converted into electrical signals one of which is selected as a received electric signal.
Here, the working transmitting and receiving module 111 appropriately variably attenuates transmitting signal light that is to be output from the light transmitting and receiving circuit 116 such that the light levels of respective channels serving as WDM light after being amplified in the light amplifier 115 are equalized. In addition, the transmitting and receiving module 111 appropriately variably attenuates receiving signal light which includes lights of respective channels and whish is transmitted through the WDM transmission path 101 such that the photodiodes of the transmitting and receiving modules 111 can receive the lights at appropriate reception levels.
The working transmitting and receiving module 111 adjusts levels of receiving signal light and transmitting signal light through the use of non-illustrated variable optical attenuators (VOAs). A VOA is generally controlled to have a larger amount of attenuation in accordance with increase in a controlling electric signal (e.g., a current signal) and has an amount “0” of attenuation when no electric signal is supplied to the VOA. There is a large variation in relationship between the largeness of a controlling electric signal and the largeness of an amount of attenuation among individual VOAs as illustrated in FIG. 7. Accordingly, the respective VOAs included in the duplexed transmitting and receiving modules 111 do not always have the same relationship between the largeness of a controlling electric signal and the largeness of an amount of attenuation.
For the above, each transmitting and receiving module 111 includes a variable optical attenuator (VOA) interposed between the light transmitting and receiving circuit 116 and the light switch 117, and carries out feedback control based on output light power from the VOA such that the amount of attenuation corresponding to the controlling electric signal is stably in a desired level. Furthermore, in order to variably attenuate receiving signal light to a desired level, a VOA is disposed on a light path upstream of the transmitting and receiving module 111, so that feedback control is carried out on the amount of attenuation in the VOA on the basis of electric signal received by and then output from the photodiode of the transmitting and receiving module 111.
The configuration in which the switch 117 of the transmitting and receiving modules 111 switches between the working module and the protection module requires only light couplers 113a and 113b, functioning as passive devices to arrange the light path to and from respective transmitting and receiving modules 111 for the redundancy multiplexing/demultiplexing section 113, so that a failure rate of the redundancy multiplexing/demultiplexing section 113 is reduced. Incorporation of active devices, such as laser diodes, having relatively high failure rate into the light transmitting and receiving sections 112 makes it possible to replace such active device without affecting the other channels being working.
Besides, the Patent Literature 1 is listed as a technique related to the present invention.    [Patent Literature 1] Japanese Patent Publication No. 2004-23295
However, since no receiving signal light is input into a transmitting and receiving module 111 which is installed but is for a channel not being used, the functional part to control an amount of attenuation of the VOA of the transmitting and receiving module 111 does not obtain the relationship between a controlling electric signal and an amount of attenuation of the VOA to be controlled even through the above feedback control. As a consequence, under the circumstance where no received signal is input, it is difficult to stably control the VOA to have a desired amount of attenuation.
In this case, for example, assuming that an unused transmitting and receiving module 111 comes into a state of being used or that the light transmitting and receiving section 112 is replaced with another section, the relationship between the largeness of a controlling electric signal and an amount of attenuation of the VOA to be controlled is not grasped at the start of using. For this reason, it may be difficult to cause the photodiode of the light transmitting and receiving circuit 116 to obtain light in an appropriate reception range at the start of receiving signal light.
As an example, the light transmitting and receiving section 112 is assumed to have the transmitting and receiving modules 111 which function as a duplex component and which include variable optical attenuators 121 and 122, as illustrated in FIG. 8. The light transmitting and receiving section 112 of FIG. 8 includes variable optical attenuators (VOA1, VOA3) each of which is interposed between the light transmitting and receiving circuit 116 and the optical switch 117 of one of the transmitting and receiving modules 111, so that transmitting signal light is variably attenuated. Furthermore, the light transmitting and receiving section 112 includes variable optical attenuators (VOA2, VOA4) each of which is included in one of the transmitting and receiving modules 111, so that receiving signal lights to be input into the transmitting and receiving modules 111 are variably attenuated. It should be noted that the variable optical attenuators 121 and 122 have different relationships between a controlling electric signal and an amount of attenuation as described above.
A VOA controller 125 that controls the variable optical attenuator 121 as illustrated in FIG. 8 obtains, from light coupler 123 and light monitors (PD1 and PD3) 124, power information of transmitting signal light that is to be output from the variable optical attenuator 121, and carries out feedback control on the amount of attenuation for the variable optical attenuator 121 corresponding to the controlling electric signal to the variable optical attenuator 121 on the basis of the output light power from the variable optical attenuator 121.
A VOA controller 127 that controls the variable optical attenuator 122 obtains, from photodiodes (PD2 and PD4) 116a of the light transmitting and receiving circuit 116, power information of receiving signal light that is to be output from the variable optical attenuator 122 and carries out feedback control on the amount of attenuation for the variable optical attenuator 122 corresponding to the controlling electric signal to the variable optical attenuator 122 on the basis of the output light power from the variable optical attenuator 122, so that the light level of the receiving signal light to be input into the photodiode 116a is stabilized.
However, when the light transmitting and receiving section 112 of FIG. 8 comes into a state of being used from the unused state or restarts the operation after replacement of parts, the light transmitting and receiving section 112 does not receive receiving signal light as illustrated in FIG. 9. Therefore, the VOA controller 127 does not grasp (has not retrieved) the relationship of the variable optical attenuator 122 to be controlled between the amount of attenuation and the controlling electric signal so that it may be difficult for the photodiode in the light transmitting and receiving circuit 116 to attenuate receiving signal light to have a receiving level in a proper range at the start of inputting receiving signal light.